The present invention relates to devices for silencing a flow of gas such as exhaust gasses originating from a combustion device, a method for silencing such a flow, a vehicle comprising one or more such devices and a stationary power generating installation comprising one or more such devices. The invention also relates to a system comprising an engine and a silencer, as well as a method for operating such a system, in which method the combination of an efficient sound attenuation and a low back pressure is used to achieve hitherto unattainable advantages with respect to engine performance and economy.
While a number of silencer designs are known, most of these are not particularly beneficial with respect to flow dynamic properties. As a result of intensive studies of the flow dynamic behaviour and requirements of silencer systems, the invention provides both basic physical principles to be complied with by silencer designs in order to obtain hitherto unattainable combinations of effective noise damping, low back pressure (pressure drop across the silencer device) and small sizexe2x80x94and specific novel mechanical design features, such as physical conformations of the passages or bodies involved in the flow pathxe2x80x94which co-operate with a suitable overall design to provide superior combinations of performance results.
It is well known within the art to silence a flow of gas by directing the flow into an inlet passage to a container, through one or more chambers in said container intercommunicating by means of passages, through a diffuser associated with one of said passages and into an outlet passage from said container.
In the case of tailor-made solutions for one-off installations or very small production series, application of the traditional method has not been able to provide optimal solutions except in exceptional cases where the element of luck has been a factor. This is due to the fact that the economical and practical possibilities for carrying out experiments and consequent design and/or dimensioning modifications and changes are not at hand.
Furthermore, the large number of parameters and considerations having implications for the sound attenuation in a silencing device have in the past prevented those skilled in the art from designing and dimensioning such a device simply and reliably in such a manner that a desired sound attenuation with an acceptable loss of pressure through the device and acceptable overall dimension were consistently achieved.
One aspect of the present invention, to be discussed in greater detail in the following, relates to a device having a curved passage for interconnecting an acoustic chamber with another acoustic chamber or with an exterior volume. A number of devices having curved passages are known. Thus, U.S. Pat. No. 4,317,502, U.S. Pat. No. 4,635,753, DE 463 156, DE 467 515, DE 570 630, DE 614 930, U.S. Pat. No. 4,579,195, U.S. Pat. No. 3,692,142, DE 26 12 421, DE 27 15 053, U.S. Pat. No. 4,126,205, CH 313 645, U.S. Pat. No. 4,046,219, DE 736 635, SU 165 634, DE 557 140 and DE 469 259 disclose various types of devices having one or more curved passages.
It is characteristic of the devices according to the present invention that they are based on design principles which are aimed at obtaining desired sound attenuation characteristics and at the same time low or very low back pressures. The design principles involve observing certain general physical/mathematical principles in connection with the particular given parameters relating to at least space constraints and the design, the design including of one or more passages leading the flow into and/or out of one or more chambers of the device, as well as one or more diffusers diffusing at least a part of the gas flow through one or more of the passages. Important parameters in this regard are the number and extent of changes of geometric configuration and arrangement and the relative dimensions of one ore more chambers and the passage or passages connecting the chambers or connecting chambers with an exterior volume. as well as the number of changes of cross-sectional are of the gas flow and the values of the individual changes in cross-sectional area. Hereby, very interesting silencer designs have been obtained which combine small size with effective sound attenuation and low back pressure.
One aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising:
a casing,
at least one acoustic chamber and contained in the casing, said chamber being through-flowed by gas,
at least one inlet pipe for leading gas into one of said at least one acoustic chamber,
at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber,
optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber,
said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area ai and a relatively higher cross-sectional area Ai,
the device fulfilling the following criteria:
(i) the average sound attenuation {overscore (xcex94dB)} conferred by each transition of cross-sectional area, approximated by the following expression:                                                         Δ              ⁢                              xe2x80x83                            ⁢              dB                        _                    =                                    1              n                        ⁢            k            ⁢                                          ∑                                  i                  =                  1                                n                            ⁢                                                log                  10                                ⁢                                                      A                    i                                                        a                    i                                                                                      ,                            (        1        )            
n being the total number of transitions of cross-sectional area of the device, Ai being the relatively higher cross-sectional area at the ixe2x80x2th transition of cross-sectional area of the gas flow, ai being the relatively lower cross-sectional area at the ixe2x80x2th change of cross-sectional area of the gas flow, k being a constant of the value 6.25 dB,
is at least
8.0 dB when the device comprises no more than two acoustic chambers,
6.0 dB when the device comprises three acoustic chambers,
5.0 dB when the device comprises 4 or more acoustic chambers,
(ii) the pressure loss across each acoustic chamber expressed as the dimensionless parameter xcex6jxe2x80x2, defined as the ratio between the static pressure loss over the chamber and the dynamic pressure at a location in said passage:                                           ζ            j            xe2x80x2                    =                                    Δ              ⁢                              xe2x80x83                            ⁢                              p                j                                                                    1                2                            ⁢              ρ              ⁢                              xe2x80x83                            ⁢                              u                2                                                    ,                            (        2        )            
xcex94pj being the static pressure loss over the jxe2x80x2th chamber, exclusive of the static pressure loss over a monolith optionally comprised in the jxe2x80x2th chamber, xcfx81 being the density of the gas at said location, u being a velocity of the gas at said location, preferably the mean gas velocity,
is at the most 1.0.
Another aspect of the invention is the above-mentioned aspect comprising a curved passage connecting two acoustic chambers or connecting an acoustic chamber with an exterior environment. This aspect can be defined as a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising:
a casing,
at least one acoustic chamber contained in the casing, said chamber being through-flowed by gas,
at least one inlet pipe for leading gas into one of said at least one acoustic chamber,
at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber,
optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber,
said device showing at least two transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area ai and a relatively higher cross-sectional area Ai,
at least one passage selected from said at least one passage being curved,
wherein the following applies to at least one selected chamber selected from said at least one acoustic chamber:
the mean cross-sectional area Aj of each of said selected chamber is at least four times the largest of:
the sum of all cross-sectional areas of passages leading gas to the selected chamber, a1, and
the sum of all cross-sectional areas of passages leading gas from the selected chamber, a2,
the mean cross-sectional area, Aj, being defined as the mean value of all cross-sectional areas along a mean trajectory for sound waves travelling from across the selected chamber,
the volume of the at least one selected chamber, Vj, is at least 8({square root over ((a1+a2)/2)})3, a1 and a2 being defined as stated above,
the cross-sectional area, Aj, and the volume, Vj, including any elements belonging to sound absorptive material inside the selected chamber and any other parts which are in acoustic communication with the selected chamber,
said at least one passage being curved having an acoustically effective length L which is at least 1.50 times the direct and straight distance in space between passage inlet and passage outlet, such as at least 1.75 times, such as at least 2 times or more, such as at least 3 times, at least 4 times, at least 5 times or at least 6 times, the direct and straight distance in space between passage inlet and passage outlet.
As will be discussed in greater detail in the following, at least part of the at least one curved passage will preferably extend over an angle of more than 180 degrees. Whenever extension of the curved passage over an angle is discussed herein, it is to be understood as the total angular turn of the curved passage, i.e. the sum of absolute turn angles which the curved passage defines. The curved passage is preferably closer to the envelope part of the silencer or of a chamber than to the center part of the silencer or the chamber, i.e., the straight-line perpendicular distance from the center line of the curved passage to the center axis of the silencer is preferably longer than the straight-line distance from the center line of the curved passage to the envelope.
The at least one passage being curved will often have an acoustically effective length L which is at least 2 times the direct and straight distance in space between passage inlet and passage outlet.
At least part of the at least one curved passage may extend over an angle of more than 180 degrees and closer to the envelope part of the silencer or of a chamber than to the center part of the silencer or the chamber, and in many embodiments adjacent to the envelope part of the silencer or of a chamber.
When the at least one curved passage connects a chamber with the exterior environment, it preferably occupies a volume which is smaller than the volume of said chamber, and when the at least one curved passage connects two chambers, it preferably occupies a volume which is smaller than the volume of the smaller of the two chambers.
In all practical embodiments of this devise envisaged so far, substantially all cross-sections of said at least one curved passage are of such a shape that the cross-section in no direction extends over the full extension of the silencer in the particular direction, and substantially no cross-section of the at least one curved passage is of such a shape that the ratio between the smallest cross-section dimension and the largest cross-section dimension is very small. Thus, substantially no cross-section of the at least one curved passage is of such a shape that the ratio between the smallest cross-section dimension and the largest cross-section dimension is 1/50 or less. The ratio between the smallest cross-section dimension of the at least one curved passage and the largest cross-section dimension of the passage is at least 0.1.
Another aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising:
a casing,
at least two acoustic chambers contained in the casing, said chambers being through-flowed by gas,
at least one inlet pipe for leading gas into one of said at least two acoustic chambers,
at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least two acoustic chambers to another of the at least two acoustic chambers or to an exterior environment or an exterior chamber,
optionally one or more monolithic bodies comprised in each of one or more of said at least two acoustic chambers,
said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area ai and a relatively higher cross-sectional area Ai,
said inlet pipe being continued by a flow deflecting element causing the gas to flow with a radial component and for causing pressure recovery to the flow upstream of a first one of said at least two acoustic chambers, wherein said passage comprises at least one diffuser for leading the gas flow into a second one of said at least two acoustic chambers, the diffuser of said passage being of a different type than a radial diffuser,
wherein the average sound attenuation {overscore (xcex94dB)} conferred by each transition of cross-sectional area, approximated by the following expression:                                                         Δ              ⁢                              xe2x80x83                            ⁢              dB                        _                    =                                    1              n                        ⁢            k            ⁢                                          ∑                                  i                  =                  1                                n                            ⁢                                                log                  10                                ⁢                                                      A                    i                                                        a                    i                                                                                      ,                            (        1        )            
n being the total number of transitions of cross-sectional area of the device, Ai being the relatively higher cross-sectional area at the ixe2x80x2th transition of cross-sectional area of the gas flow, ai being the relatively lower cross-sectional area at the ixe2x80x2th change of cross-sectional area of the gas flow, k being a constant of the value 6.25 dB,
at least
6.0 dB when the device comprises no more than two acoustic chambers,
5.0 dB when the device comprises three acoustic chambers,
4.0 dB when the device comprises 4 or more acoustic chambers.
A device of the above-mentioned type is particularly advantageous with respect to low-frequent sound attenuation as the pressure drop across the device is low in comparison to prior art devices,
Preferably, the local natural frequency, fe, of at least one system comprising the gas of two consecutive acoustic chambers ACj and ACj+1 and the gas of the passage interconnecting said two acoustic chambers, approximated by the following expression:                                           f            e                    =                                    c                              2                ⁢                                  xe2x80x83                                ⁢                π                                      ⁢                                                            a                  L                                ⁢                                  (                                                            1                                              V                        j                                                              +                                          1                                              V                                                  j                          +                          1                                                                                                      )                                                                    ,                            (        3        )            
Vj and Vj+1 being the volumes of the chambers ACj and ACj+1 respectively (the volume Vj+1 being set to infinite when the chamber ACj is connected to an exterior environment or an exterior chamber in a downstream direction), a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity,
is at the most 0.75 times a characteristic frequency of the flow system. For many applications of the device, it is preferred that the local natural frequency, fe, is at the most 0.5 times the characteristic frequency of the flow system, such as 0.4 times or 0.3 times or even 0.25 times, such as 0,2 times, 0.15 or 0.1 or even lower. However, also an interval between 0.75 and 0.95 may be interesting.
Preferably the value xcex6jxe2x80x2 of each acoustic chamber is at the most 1.0. The value of xcex6jxe2x80x2 of at least one acoustic chamber is preferably at the most 0.75, or even lower such as 0.5, 0.25, 0.2 or even lower. Special designs of the device allow for a value less than or equal to 0, as will be discussed below.
A device according to the invention may comprise one or more radial diffusers and/or one or more axial diffusers and/or one or more circular conical diffusers and/or one or more annular diffusers and/or a plurality of conical diffusers arranged on a substantially cylindrical surface and/or one or more diffusers for reversing the direction of flow and/or one or more double diversion diffusers, at least some of the above-mentioned diffuser types being known per se. Any other diffuser types known per se may be applied.
Each of the at least one acoustic chamber may be substantially cylindrical, and one or more outlets from said at least one diffuser may be located substantially at the axial centre of the chamber associated with said diffuser. When the chamber is substantially cylindrical it defines a cylinder axis. Preferably, one or more outlets from said at least one diffuser are located at a distance from the cylinder axis of approximately two thirds of the radius of the acoustic chamber, so as to obtain fixation of pressure nodes. This principle, known per se, is described in detail in European patent 0 683 849.
It should be understood that the above performance criteria may be fulfilled by all chambers and all passages of the device by a device according to the invention.
The sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate is preferably less than 5 dB(A) higher than the self-generated noise of a circular cylindrical reference chamber through-flowed at said gas flow rate, the cross-sectional area of the inlet passage leading gas into said acoustic chamber being a1, the cross-sectional area of the passage leading gas from said acoustic chamber being a2, said reference chamber:
being of the same volume as each of said at least one acoustic chamber,
having a length equal to its diameter,
having flat end caps,
being provided with centrally positioned holes in its flat end caps,
having a first end cap which is connected to a cylindrical inlet pipe of a cross-sectional area which is approximately equal to a1, the terminating surface of said cylindrical inlet pipe being aligned with said first end cap,
having a second end cap which is connected to a cylindrical outlet pipe of a cross-sectional area which is approximately equal to a2, said cylindrical outlet pipe having a rounded inner edge at its interconnection with said second end cap and being aligned with said second end cap.
The sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate may be less than 4 dB(A) higher than the self-generated noise of the reference chamber, or even less than 3 dB(A), such as 2 dB(A) or 1 dB(A). A sound level of self-generated noise which is less than the self-generated noise of the reference chamber may even be achieved with a device according to the invention.
In a device according to the invention, the distance between an inlet to a chamber and the inlet to a passage is preferably so large that substantially no unstable flow occurs in the chamber.
According to the invention, the generatrix of at least part of at least one curved passage selected from said at least one passage may be wound in a peripheral direction, at least part of the curved passage having a plane spiral form. The generatrix of at least a part of at least one curved passage selected from said at least one passage may be wound in a peripheral direction, said part of said curved passage extending in a longitudinal direction, so as to form a screw-like helical form. Thereby, all three dimensions of space are utilized in order to achieve a relatively long passage or relatively long passages.
By winding the connecting passage, i.e. by utilizing the third spatial dimension, the length of the passage may be significantly increased, the natural frequency of the silencing device thus being decreased, cf. equation (3). The flow of the passage may constitute a flow cross sectional area increase in the flow direction. Thus, a diffusing effect may be obtained for static pressure recovery. The cross sectional area increase may be two- or three-dimensional. The passage may have any cross sectional shape, such as rectangular, circular, ellipsoidal or any other shape.
The curved part of the passage may extend over an angle between 0xc2x0 and 90xc2x0, or over an angle between 90xc2x0 and 180xc2x0, or over an angle between 180xc2x0 and 270xc2x0, or over an angle between 270xc2x0 and 360xc2x0, or over an angle between 360xc2x0 and 720xc2x0, or over an angle of 720xc2x0 or more.
The device according to the invention may comprise at least two acoustic chambers, wherein the curved passage interconnects two chambers, a first of which surrounds a second one, the second chamber thus being xe2x80x98embeddedxe2x80x99 in the first one.
The generatrix of the curved part of the passage may extend along a surface of revolution, so as to define itself a surface of revolution. The surface of revolution may have any shape, e.g., conical.
At least one monolithic body or a monolith such as a catalyst or a particle filter, may be positioned upstream or downstream of an inlet passage and/or an outlet passage of the one or more chambers, in some embodiments the monolithic body may be positioned substantially immediately upstream or substantially immediately downstream of said inlet passage and/or said outlet passage of the one or more chambers. The monolithic body may be of an annular form.
In the present context, the term xe2x80x9cmonolithic bodyxe2x80x9d or xe2x80x9cmonolithxe2x80x9d designates, as is customary in the art, a body of an overall or macroscopic monolithic appearance, often a cylindrical body, which has a structure allowing an overall axial gas flow through the body. The term xe2x80x9cmonolithicxe2x80x9d does not rule out that the body could be made from a plurality of segments joined or arranged together. The structure allowing an overall axial gas flow through the body will depend on the construction and material of the monolith; two typical relevant monolith types are:
a monolith made from a corrugated foil wound up cylindrically so that the corrugations provide axial gas flow channels, and
a monolith made of a particulate ceramic material, e.g., silicon carbide particles sintered together, and having a honeycomb structure comprising axial channels constituted by a plurality of coextending throughgoing passages separated by common passage walls, the passages being closed at the inlet and the outlet end, alternately. Thus, in a filter body of this kind, the gas travels into the passages open at the inlet side, through the walls into the passages open at the outlet side and then out of the filter body.
Monoliths are sometimes inserted into silencers so as to combine silencing with gas purification, either in catalytic processes, in mechanical filtering, or in both. In most cases such monoliths are placed within one or more chambers of the silencer. Monoliths can provide significant silencing at medium and high frequencies, but less silencing at low frequencies. Obviously, monoliths cause added pressure drop to the piping system.
In case the purification relies solely on catalysis, the monolith is usually made as a honeycomb structure with straight channels, termed a through-flow monolith. The walls are thin, so that the open frontal area is typically 70-90%, depending mainly on the material (ceramic, metal, etc.).
Alternatively, a monolith may be made as a wall-flow monolith, i.e. the channels are perforated and partly blocked, so that the gas flow is forced to pass through those perforations, describing a tortuous pathway through the monolith. Such a monolith is used either for pure filtering or for combined filtering and catalytic treatment of the gas. Sometimes the open frontal area becomes less than 70%. Wall-flow monoliths cause pressure drops which are substantially higher than pressure drops of through-flow monoliths.
The silencing effects of monoliths can roughly be described as follows:
1. The flow within the thin channels causes high viscous friction which dampens mainly medium and high frequencies.
2. The porosities of the channels provide an aggregate acoustic volume which adds to the volume of the chamber in which the monolith is placed.
3. At the entrance and at the exit of the monolith there is an effective change in cross-sectional area which causes sound reflection, in the same way as occurs at flow entrances and flow exits connecting silencer chambers to passages. However, the relative change in cross sectional area is normally much smaller in the case of monoliths, in particular in connection with through-flow monoliths.
Since monoliths are usually fixed to the casing by an annular ring element, the effective change in cross section usually is somewhat bigger than what is given by the frontal area percentage of the monolith as such. This percentage is referred to as the effective frontal area percentage.
On the basis of this understanding, monoliths are handled in the following way as elements of silencers designed and dimensioned according to the invention:
when the effective frontal area percentage of the monolith placed in a chamber is bigger than approximately 50%, the porosity of the monolith is regarded as an extension to the chamber volume,
when the effective frontal area percentage of the monolith is smaller than approximately 50%, the monolith is treated as a connecting passage with an effective cross sectional area roughly equalling the sum of cross sectional areas of all channels within the monolith,
the pressure drop across the monolith is added to the pressure drop of the silencer without the monolith, i.e. a silencer having the same dimensions and geometry, but without the monolith. This means that when dimensioning a silencer for a given total sound attenuation, SDB, and for a given pressure drop, SDP, the pressure drop across the monolith should be subtracted from SDP to determine the residual pressure drop at disposal for silencer design.
For given flow velocities and gas temperatures, approximate pressure drops across monoliths can be calculated on the basis of formulae and experimentally based constants given in literature. More precise predictions can be made on the basis of data provided by manufacturers, or in rather simple laboratory experiments.
At least one pipe or passage may be annular, constituted by an inner cylinder and by an outer cylinder. The annular pipes or passages may be provided with means, such as e.g. walls, for segmenting the annular passage into a number of sub-passages having a rectangular or circular cross sectional outline or any other cross sectional outline. Thereby, rotating stall phenomena may be eliminated or at least reduced.
At least one of the at least one pipe or passage which is annular may be a passage connecting two chambers. The annular passage may diffuse at least part of the gas flow directed therethrough. The at least one pipe or passage being annular may thus constitute a flow cross sectional area increase in the flow direction. By applying an annular passage constituting a cross sectional area increase it is possible to achieve a relatively large cross sectional area increase over a relatively short longitudinal distance while avoiding flow separation in the passage. Thus, a relatively large pressure recovery may be achieved over a relatively short distance which is important, e.g., for applications where the available space is limited, e.g., in vehicles such as trucks.
The annular passage may comprise a constant flow area part and an outlet diffuser part. The constant flow area part contributes to the length of the connecting passage.
The inner cylinder may extend into said first chamber by a cylinder of substantially the same diameter as said inner cylinder, and said outer cylinder may be connected to a flow-guiding body with a curvature, so as to obtain optimal flow conditions through the device.
Sound absorptive material is preferably contained within said cylinder and/or within a continuation cylinder extending into said first chamber and/or within a continuation cylinder extending into said second chamber. Obviously, one aim of providing sound absorptive material is to reduce the sound level of the gas flow. Though in preferred embodiments, the sound absorptive material is comprised within said cylinders, it may additionally/alternatively be comprised at the outer periphery of the surrounding casing. Preferably, at least some of the sound absorptive material communicates with the gas flow, e.g., through a perforated wall. Thus, at least part of the continuation cylinder may be perforated. It is preferred to apply said cylinders which at least partly separate the sound absorptive material from the gas flow in order to avoid that the sound absorptive material is being damaged by the gas flow. At locations of cross-sectional increase or decrease or in the vicinity of such locations, the walls are preferably non-perforated so as to avoid damaging of the sound absorptive material and/or so as to avoid undesired flow perturbations which may increase pressure loss or generate turbulence.
In a preferred embodiment, the outflow from said connecting passage passes into an annular passage inside said second chamber, said annular passage being made up of at least a perforated portion of an inner cylinder and an outer, perforated cylinder, both said cylinders separating sound absorptive materials from gas flow within said second chamber. The outflow from the connecting passage may pass directly into an annular passage.
In order to obtain optimal flow conditions in the device, unstable flow conditions in the devices according to the invention should be avoided. Thus, for example, the distance between the inlet to the first chamber and the inlet to the annular passage should be so large that essentially no unstable flow occurs in the first chamber.
With the aim of preventing unstable flow in the first chamber and/or allowing for a rather long passage, the distance may be at least 2% larger than the distance below which unstable flow would occur. Preferably, said distance should be at least 5% larger than the distance below which unstable flow would occur, normally at least 10% larger. When the total length of the device is limited, it is not desired that the distance is more than 50% larger than the distance below which unstable flow would occur, however for some applications the distance may exceed said 50%.
The device according to the invention is adapted for being connected to a flow system, e.g., the exhaust system of a vehicle comprising an internal displacement engine and/or a turbo machine or it may be suited for being connected to the exhaust system of a stationary power generating installation comprising an internal displacement engine and/or a turbo machine.
In the present context, the vehicle may be any vehicle, such as e.g., a diesel engine powered truck, bus, a petrol or diesel driven car, a railway locomotive, an airplane, such as a piston engine powered air plane, a military vehicle, such as a tank, a petrol, a gasoline or a gas engine powered truck, bus, car or any other moveable, engine driven device. The vehicle may also be any ship or boat having a combustion device. The diesel engine may be a spark ignited or compression ignited diesel engine.
The stationary power generating installation may be a power station having one or more gas turbines driven by flow originating from suitable combustion means, such as, e.g., one or more boilers, fuel engines or other combustion means.
One major benefit of a device according to the invention is a significant reduction of pressure loss over the device compared to known devices. The reduction of pressure loss over the device reduces the fuel consumption of the combustion device and increases the power generated by the combustion device at a given fuel consumption. The pressure drop may be expressed as the dimensionless parameter xcex6 being defined as the ratio between the pressure loss over the device and the dynamic pressure at an appropriate location in the device or adjacent to the device, i.e.:                     ζ        =                              Δ            ⁢                          xe2x80x83                        ⁢            p                                              1              2                        ⁢            ρ            ⁢                          xe2x80x83                        ⁢                          u              2                                                          (        4        )            
where:
xcex94p is the pressure drop over the device,
xcfx81 is the density of the gas at said location,
u is a velocity of the gas at said location, preferably the mean gas velocity.
An appropriate location could be, e.g., the inlet pipe, the outlet pipe, a location upstream of the inlet pipe, a location downstream of the outlet pipe, or any appropriate position inside the device where the flow velocity corresponds to the gas flow rate originating from the combustion device. As will be illustrated in the example below, the invention provides a device for silencing a gas flow, the device having a low xcex6-value.
In most embodiments of the invention, xcex6 will be lower than 10. Typically, it will be between 0.5 and 4.
The pressure drop across a silencer of a given type is typically roughly proportional to the number of chambers. Therefore, when analyzing pressure drops, it is expedient to focus the xcex6-value per chamber, xcex6xe2x80x2, defined as:                                           ζ            xe2x80x2                    =                                                    p                1                            -                              p                2                            -                              Δ                ⁢                                  xe2x80x83                                ⁢                                  p                  m                                                                                    1                2                            ⁢              ρ              ⁢                              xe2x80x83                            ⁢                              u                2                2                                                    ,                            (        5        )            
where:
p1 is the static pressure at a suitable location in the chamber inlet passage(s),
p2 is the static pressure at a suitable location in the chamber outlet passage(s),
xcex94pm is the static pressure drop across a monolith optionally comprised in the chamber,
u2 is the mean flow velocity in the outlet passage(s),
the suitable location being, e.g., halfway between passage inlet and outlet in case of a passage connecting two chambers, substantially immediately upstream of the first chamber in case of an inlet passage to the silencer (unless when the inlet passage extends into the silencer and shows a decrease of cross-sectional area for the gas flow; in that case p1 is the static pressure taken at the most upstream position where the mean flow velocity reaches a maximum), and substantially immediately downstream of the last chamber in case of an outlet passage of the silencer.
One reason for subtracting the pressure drop across a possible monolith is that such an element normally contributes only to a small extent to low frequency sound attenuation while causing a substantial pressure drop in addition to the pressure drop across the chamber. Provided the monolith is inserted in an appropriate way, the pressure drop across a silencing chamber having a monolith comprised therein may be expressed as the sum of the pressure drop across the monolith and the pressure drop across the chamber when having no monolith comprised therein.
In a preferred embodiment of the invention, xcex6xe2x80x2 is below 1.5. In further embodiments it may be lower than 1.0, or lower than 0.5, or even lower than 0. Negative values can be achieved when a diffuser is fitted onto the chamber inlet passage, and the flow cross-sectional area of an outlet passage is substantially larger than the flow cross-sectional area of an inlet passage, so that chamber pressure drop is rated against a rather small dynamic pressure prevailing in the outlet passage. In embodiments of the invention, xcex6xe2x80x2 is usually below 1.0, when flow cross-sectional areas of passages are of equal or almost equal size.
The combustion device/means mentioned in the present text may be an internal combustion engine, such as a diesel, petrol, gasoline or gas engine, e.g. a two or four stroke piston engine, a Wankel engine or a gas turbine connected to a boiler or any other appropriate combustion or energy extracting device, e.g., the combustion system of a stationary power generating installation, such as power station.
A suitable method for dimensioning the devices according to the invention comprises approximating:
(I) the local natural frequency, fe, of one or more mass systems comprising the gas mass of two consecutive acoustic chambers, ACj and ACj+1, by applying the following expression:                                           f            e                    =                                    c                              2                ⁢                                  xe2x80x83                                ⁢                π                                      ⁢                                                            a                  L                                ⁢                                  (                                                            1                                              V                        j                                                              +                                          1                                              V                                                  j                          +                          1                                                                                                      )                                                                    ,                            (        3        )            
Vj and Vj+1 being the volumes of the chambers ACj and ACj+1 respectively (the volume Vj+1 being set to infinite when the chamber ACj is connected to an exterior environment or an exterior chamber in a downstream direction), a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity,
(II) the sound attenuation xcex94dBi conferred by one or more of the at least one transition of cross-sectional area by applying the following expression:                                           Δ            ⁢                          xe2x80x83                        ⁢                          dB              i                                =                      k            ⁢                          xe2x80x83                        ⁢                          log              10                        ⁢                                          A                i                                            a                i                                                    ,                            (        6        )            
Ai being the relatively higher cross-sectional area at the ixe2x80x2th transition of cross-sectional area of the gas flow, ai being the relatively lower cross-sectional area at the ixe2x80x2th transition of cross-sectional area of the gas flow, and k is an empirically determined constant
for adapting two or more of the following parameters relative to each other:
(a) the length L and the representative cross-sectional area a of one or more of the at least one passage,
(b) the number of changes, n, of transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area ai and a relatively higher cross-sectional area Ai,
(c) the number of acoustic chambers, m, comprised in the casing,
(d) the relatively lower cross-sectional area ai (i=1 . . . n) and the relatively higher cross-sectional area Ai (i=1 . . . n) of each one of the n changes of cross-sectional area,
(e) the volume or volumes, Vj (j=1 . . . m), of each one of the m acoustic chambers comprised in the casing,
(f) the natural frequency, fe, of one or more domains of the device comprising two consecutive acoustic chambers ACj and ACj+1, or
(g) the total sound attenuation conferred by the device.
The adaptation of the parameters:
(a) the length L and the representative cross-sectional area a of each one of the at least one passage,
(b) the number of changes, n, of cross-sectional area for the flow of the gas between the relatively lower cross-sectional area ai and the relatively higher cross-sectional area Ai,
(c) the number of acoustic chambers, m, comprised in the casing,
(d) the relatively lower cross-sectional area ai (i=1 . . . n) and the relatively higher cross-sectional area Ai (i=1 . . . n) of each one of the n changes of cross-sectional area,
(e) the volume or volumes, Vj (j=1 . . . m), of each one of the m acoustic chambers comprised in the casing,
(f) the natural frequency, fe, of any domain of the device comprising two consecutive acoustic chambers ACj and ACj+1,
(g) the total sound attenuation xcexa3xcex94dB conferred by the device,
relative to each other may be performed by performing the steps of:
(i) restricting the values of one or more of the parameters (a)-(g) by assigning a predetermined value or a predetermined limit to each one of said one or more parameters,
(ii) assigning values to the parameters a, L, ai and Ai if predetermined values have not been assigned to those parameters at step (1),
(iii) approximating the local natural frequency, fe, of any domain of the device comprising two consecutive acoustic chambers ACj and ACj+1 by applying the following expression:                                           f            e                    =                                    c                              2                ⁢                                  xe2x80x83                                ⁢                π                                      ⁢                                                            a                  L                                ⁢                                  (                                                            1                                              V                        j                                                              +                                          1                                              V                                                  j                          +                          1                                                                                                      )                                                                    ,                            (        3        )            
xe2x80x83Vj and Vj+1 being the volumes of the chambers ACj and ACj+1 respectively (the volume V2 being set to infinite when the chamber ACj is connected to an exterior environment or an exterior chamber in a downstream direction), a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity,
(iv) if a value or a limit for the local natural frequency has been prescribed at step (i), comparing said value or limit to fe as determined at step (iii),
(v) approximating the total sound attenuation xcexa3xcex94dB conferred by the device by applying the following expression:                               ∑                      Δ            ⁢                          xe2x80x83                        ⁢            dB                          =                  k          ⁢                                    ∑                              i                =                1                            n                        ⁢                                          log                10                            ⁢                                                A                  i                                                  a                  i                                                                                        (        7        )            
xe2x80x83n being the total number of changes of cross-sectional area of the gas flow, Ai being the relatively higher cross-sectional area at the ixe2x80x2th change of cross-sectional area of the gas flow, ai being the relatively lower cross-sectional area at the ixe2x80x2th change of cross-sectional area of the gas flow, and k is an empirically determined constant,
(vi) if a value or a limit for the total sound attenuation of the device has been prescribed at step (i), comparing said value or limit to xcexa3xcex94dB as determined at step (v),
(vii) if the comparisons performed at steps (iv) and (vi) reveal that the limitations prescribed at step (i) are not fulfilled, updating the values of the parameters set at step (ii) and repeating steps (iii)-(vii) until the limitations prescribed at step (i) are achieved.
Expressions known per se may be used for approximating the increase or decrease of static pressure at one or more of said at least one transition of cross-sectional area.
A predetermined limit or value may be assigned to the pressure loss over the device at step (i), and repeating steps (iii)-(vii) until said predetermined limit or value for the pressure loss over the device is achieved.
Steps (iii)-(vii) above may be repeated until the above-mentioned parameters are mutually optimized.
The value of k is suitably in the range 5-7, e.g., in the range 6-6.5, such as 6.25.
When the device further comprises at least one resonance chamber with or without a therein enclosed sound absorptive material, which chamber communicates with one of said at least one acoustic chamber through at least one communication passage, the method comprising adjusting the volume of the at least one resonance chamber and the geometric configuration of the at least one resonance chamber and the communication passage or passages being so as to have a selected center attenuation frequency so as to supplement the sound attenuation achieved with said acoustic chamber or chambers at frequencies of said flow system around said center attenuation frequency.
The volume of the at least one resonance chamber may be adjustable for instance by means of displaceable adjustment means, in which case the range of the volume of the at least one resonance chamber may be adjusted so as to allow for a variable center attenuation frequency within a given range.
When the device comprises one or more resonant chambers contained in said casing with the aim of providing added sound attenuation, the method comprising designing the volumes of said resonance chambers as resonators with selected center frequencies.
According to a further aspect of the invention, a vehicle comprising an internal displacement engine and/or a turbo machine and a device according to the invention is provided, the device being comprised in the exhaust system of the vehicle.
The invention further relates to a stationary power generating installation comprising an internal displacement engine and/or a turbo machine and a device according to the invention, the device being comprised in the exhaust system of the power generating installation.